1. Technical Field
The present invention relates to a pitch for carbon fibers, and more particularly, to a method for preparing a pitch for carbon fibers, which has a high softening point through radical crosslinking, and a pitch for carbon fibers prepared by the same.
2. Description of the Related Art
Carbon fibers are classified into polyacrylonitrile (PAN)-based carbon fibers, cellulose-based carbon fibers, pitch-based carbon fibers, and phenol resin-based carbon fibers according to raw materials. Among these carbon fibers, the pitch-based carbon fibers are largely classified into mesophase pitch-based carbon fibers and isotropic pitch-based carbon fibers according to the kind of precursor pitch.
The mesophase pitch-based carbon fibers are prepared using a mesophase pitch which is optically anisotropic, as a precursor for carbon fibers, and the isotropic pitch-based carbon fibers are prepared using an isotropic pitch, which is optically isotropic, as a precursor for carbon fibers. The mesophase pitch-based carbon fibers generally exhibit high strength and high elasticity, whereas the isotropic pitch-based carbon fibers exhibit general-purpose properties such as low strength and low elasticity.
However, since the mesophase pitch-based carbon fibers are applied to a limited range such as ultra-high temperature materials and the like, there is a further need for development of an isotropic precursor pitch for preparation of general-purpose carbon fibers. Since the general-purpose carbon fibers require high strength and high elasticity despite low price thereof, more studies of initial raw materials and preparation processes are needed to improve properties thereof.
The pitch-based carbon fibers are generally prepared by fiberization of a precursor pitch through melt spinning using a spinning machine, followed by oxidative stabilization of the fiberized pitch at a temperature from about 150° C. to about 350° C. in an oxidative atmosphere for a predetermined period of time and then treatment at a temperature from about 700° C. to about 3000° C. in an inert atmosphere for a predetermined period of time.
In preparation of carbon fibers, production cost of the carbon fibers depends on price of a precursor pitch, spinnability of the precursor pitch, a rate of oxidative stabilization, carbonization yield after carbonization, and the like. In addition, an oxidative stabilization process requires a longer reaction time than any other individual preparation processes. Thus, development of the precursor pitch exhibiting excellent oxidative stabilization is known to be important.
Examples of a method for preparing an isotropic pitch, which is used as a raw material of isotropic pitch-based carbon fibers and has a softening point of about 200° C. or more, may include a method of removing low molecular weight components from a coal-based pitch through vacuum distillation and solvent extraction, a method of converting low molecular weight components of a raw material into high molecular weight components through simple thermal condensation, and a combination thereof. However, although the isotropic pitch having a relatively narrow molecular weight distribution can be prepared from a raw material having a wide molecular weight distribution by these methods, there are problems in that these methods have low yield and the pitch prepared by these methods has drawbacks in terms of homogeneity and spinnability due to mesophase components remaining upon heating.
Recently, research into use of petroleum refinement residues, which are low-priced and exhibit excellent elastic modulus and thermal and electrical conductivity, as a raw material of the isotropic pitch-based carbon fibers is being actively made. Among such residues, fluidized catalytic cracking decant oil (FCC-DO) and pyrolized fuel oil (PFO), which correspond to petroleum-based raw materials, particularly attract attention as a suitable raw material for high value-added carbon materials, such as high-strength carbon fibers, needle coke, and artificial graphite, due to a high degree of aromatization thereof and a low amount of sulfur and insolubles therein.
Korean Patent Registration Publication No. 0244912 (publication date: Feb. 25, 1999) discloses a method for preparing a high-softening point optically isotropic pitch, in which a pitch capable of being used as a precursor of carbon fibers is prepared by reacting a halogen compound and a radical initiator with a petroleum-based material used as a carbon source.